Crystals are rarely pure because they generally contain small quantities of foreign matter which have been built-in or occluded. Gases, liquids and solids are readily occluded in a growing crystal; dirt, air and mother liquor are the most common occlusions found in commercial crystals. From the commercial crystallization point of view, the main interest lies in finding methods to prevent such occlusions from occurring. Vapor occlusions are minimized by avoiding vigorous agitation or boiling. The application of ultrasonic radiation to the system is also used to prevent bubbles or particles from adhering to a growing crystal face. Most importantly, the crystallizing system is kept clean to avoid dirt, other debris and particularly organic contaminants from being occluded into a crystal.
This invention is oppositely directed; that is, this process deliberately provides fine PTFE contaminant particles, in the size range from about 0.05 micron to about 0.5 micron in diameter, as seeds to facilitate the formation and growth of desirable large crystals in which the seeds are beneficially occluded.
Crystallization comprises three fundamental steps: supersaturation (or super cooling), formation of crystal nuclei, and the subsequent growth of these nuclei into crystals. Supersaturation may be achieved by cooling, evaporation of the solvent, addition of a precipitating agent, as a result of the chemical reaction, etc., and is vital in any industrial crystallizing operation. However, supersaturation alone is not sufficient to cause crystals to grow. Before the growth stage can commence, there must exist in the system a number of tiny crystal embryos or nuclei. These may be formed spontaneously, induced artificially, or deliberately added.
One of the best known methods of inducing crystallization is that of seeding, that is, the deliberate addition of tiny crystals to the super cooled system. The seeds do not necessarily have to consist of the material to be crystallized; crystals of some isomorphous substances will often work. In some cases, inert crystalline particles can give the desired effect. Super cooled systems are sometimes seeded accidentally by atmospheric dust, but such seeding does not produce the results of seeding with finely divided PTFE.
It is also well-known that the accidental production of nuclei, referred to as `false grain`, should not be permitted in an industrial crystallizer. This is undesirable, not for the mass of material precipitated, which may be relatively small, but for the number of seeds which are produced. In any case, it is well-known that if controlled growth is required, accidental nucleation must be avoided or counter-acted, and extreme precautions must be taken to get rid of false grain where conventional precautions fail. The deliberate introduction of any contaminant particles, particularly organic particles of a synthetic polymeric resin, is contrary to recognized commercial crystallization practice.
Crystals of a given substance, produced by different methods, may be completely different in physical appearance, even though they belong to the same crystallographic system. This variation in external form is called a modification of habit and it results from the inhibition of growth in one direction or the enhancement of growth in another. The most common cause of habit modification is the presence of impurities; these are absorbed on certain crystal faces and stunt the growth in those directions. Many complex dyes act as habit modifiers for inorganic salts and their habit-modifying power depends on their anionic or cationic nature. Selected surface active agents also have been found useful for habit-modification purposes. Surprisingly, in general, the process of this invention has no noticeable stunting effect. Quite to the contrary, crystals are grown by this process which are generally uniformly well-defined and larger than crystals grown without seeding with the PTFE particles, irrespective of the particular type of industrial crystallizer used. Crystals of particular lattice structure appear to be more susceptible to the beneficial effects of this process than others, but surprisingly I have found no inorganic ionic salt crystals which are detrimentally affected by deliberate seeding, in the specified preferred ranges, with a dispersion of PTFE resin particles.
Again, it is known that various relationships have been proposed for theoretically predicting the mass deposition rates on crystal faces, and these relationships have been used to predict geometrically regular crystal shapes, the size of a crystal after it has remained in a crystallizing medium for a fixed period of time, and the like. The crystallization process of the instant invention does not appear to be characterized by any known mathematical relationship, and in fact, there appears to be no simple method of predicting with accuracy the effect of seeding even in crystals of the same lattice structure.
While it is clearly advantageous to produce crystals as large as possible, their actual size is only of secondary importance; what really matters, with relatively large crystals, is the regularity of the product. The less regular the crystals, the fewer voids there will be between crystals. Moreover, with less regular, relatively small crystals, crystalline fines are generally present which given rise to objectionable dust which makes handling the crystals in bulk a most unpleasant task. Of course, besides being essentially dust-free, uniform, large crystal masses have many other desirable properties; they can be filtered and washed more efficiently during processing, they have good flow characteristics, and they have a pleasing appearance--an important sales factor, particularly in the sale of big amounts of ammonium nitrate, ammonium sulfate, and nickel sulfate. Thus, there is a need for a simple, economical method of producing relatively large, well-defined crystals which may be grown from a supersaturated solution, and which are essentially free from dusty crystalline fines.
Again, relatively small crystals are highly susceptible to caking, even if the crystals are uniform. Thus there is also a need for a process which makes it unnecessary to provide the crystals with a coating agent such as is popular for many masses of small crystalline particles, for the purpose of retaining their free-flowing characteristics. It is common knowledge, for example, that table salt is coated with a very fine dust of magnesium carbonate. Icing sugar is coated with a tricalcium phosphate or corn flour. Other anti-caking agents that find use for industrial purposes include chalk, calcium sulfate, kaolin, diatomaceous earth and the like. Addition of anti-caking agents not only is an inconvenience and an economic burden, but generally introduces a significant level of an undesirable contaminant.
The importance of this process and its several benefits and advantages are unexpectedly due to the seeding of a saturated solution of inorganic salts and particularly ionic metal salts with an aqueous dispersion of sub-micron and micron-size particles of PTFE, as will be explained hereinafter.